This invention relates to the treatment of raw water supplies such as those used by municipalities and industries as a source of fresh water and has to do with the removal of turbidity factors and coloring matter from such supplies and also with the recovery of waste water and chemical values that are commonly discarded as waste products of the treatments accorded such supplies.
Some of the raw water supplies used by industries and municipalities, although having hardness levels which are tolerable for the intended uses, nevertheless have entrained clay and other factors of turbidity which, along with organic coloring matter, must be removed before use as a fresh water source. This is commonly done by settling and filtering procedures which involve the use of a suitable coagulant such as alum. The sludges obtained by the settling procedures are mainly composed of water, clay and the coagulant used in the treatment, and the disposal of such sludges has caused pollution problems, mainly because of the contained coagulant. The water content of the sludge is, of course, lost with the disposal of the sludge as such there is a need for procedures in such instances which would avoid environmental contamination by the coagulants used in settling the undesirable turbidity factors. Some of these raw water supplies having what may be considered as a "tolerable" hardness level actually have such a low level of hardness as to provide an environment for the excessive corrosion of equipment used in their handling. Therefore it would also be advantageous in such cases to raise the hardness level by the treatment accorded the supply.
Yet other raw water supplies have such a high level of temporary and/or permanent hardness as to require softening before use as a fresh water source. The lime treatment processes, such as the lime-soda process, are widely used for such purposes but there are certain waste disposal problems which accompany their use. The lime treatments, for example, produce a waste aqueous sludge of coprecipitated calcium carbonate and magnesium hydroxide that also contains any insoluble turbidity factors as are present in the raw water supply. In many areas this aqueous sludge is merely passed to a local body of water in which it serves as an environmental pollutant. In other areas, the sludges are collected in lagoons and other outdoor storage areas where throughout the years they have progressively taken up more and more valuable land areas. Alum and/or other coagulants are also usually used in carrying the lime processes into practice, and these coagulants and the magnesium values of the aqueous sludges also create problems of pollution in the vicinity of the storage areas due to water runoff. Since sizable amounts of treated water are lost with the disposal of the aqueous sludges along with quantities of calcium and magnesium chemicals, it would be desirable if procedures were available for eliminating or reducing the disposal problem and for recovering the valuable sludge components.
A few water treatment systems have been equipped with sludge treatment facilities for recovering a reusable and marketable lime product. The procedures used at the sludge treatment facilities involve the carbonation of the sludge to effect a phase separation between the calcium and magnesium values. The magnesium hydroxide component of the sludge is solubilized by the carbonation procedure to provide a solute of magnesium bicarbonate in the aqueous component of the sludge. The liquid phase is then separated from the residual insolubles and is disposed of as a by-product waste material while the residual insolubles are calcined to recover a lime product that is reusable in the water treatment process and also available for sale on the open market when the recovered amounts exceed the recycled lime requirements of the softening procedure. The disposal of the carbonated liquor, however, creates a pollution problem of concern to ecologists, and it also accounts for losses of potentially valuable sludge components. It would accordingly be desirable if procedures existed for the recovery of the water and magnesium components of such by-product waste.
The sludge treatment procedure for recovering the calcium values has heretofore been limited to the treatment of sludges obtained from the softening of raw waters that are basically free of turbidity factors. Clay and other turbidity factors present in the raw waters are separated as components of the sludges and carry through the sludge treatment steps with the insoluble calcium carbonate. The clay ultimately appears as a contaminant in the recovered lime product. This limits the recycled use of any recovered lime product to amounts consistent with a tolerable buildup of the recycled turbidity factors. The recovery of lime from sludges containing appreciable amounts of clay would therefore necessitate the use of large amounts of uncontaminated makeup lime for the softening procedures and would result in a low quality lime by-product having limited use in the market areas. As such, there is a need for calcium recovery procedures which provide a quality lime product and which can be used in conjunction with the treatment of sludges obtained from the softening of turbid raw water supplies.
The calcium recovery procedures currently in use have also experienced difficulty in separating the magnesium values from the calcium values. The insoluble calcium carbonate which is separated from the carbonated liquor contains magnesium values that become dead burn when the insoluble products are calcined. The recycle and reuse of this lime product is accordingly also limited by the tolerable buildup of dead burn magnesite which accompanies the recycling procedures, and needless to say the magnesite inclusion in the calcined product also limits the open market usage of the recovered product. There is accordingly a need for improved procedures for separating the magnesium and calcium values in the recovery systems.